Gears are manufactured by several different machining methods, e.g., cutting, grinding, forging, rolling, etc. In recent decades roll-forming processes have been used in the manufacture of helical-tooth gears from cylindrical rod workpieces. In these well-known rolling processes, the toothed articles are formed by the relative motion of sets of dies which themselves have helical teeth that not only displace the metal on the surface of the rods but also pull the rod through the dies during the forming process. Some processes use a pair of flat dies that are reciprocated relative to each other, while others use sets of large cylindrical dies that roll in the same direction, squeezing the workpiece rod between them to form mating teeth on the surface of the rod as the rod rotates between them like a planetary gear.
It is also well-known that such prior art rolling systems create a fissure and seam at the crest of the rolled tooth because the metal of the workpiece is squeezed radially outward faster along the tooth faces of the die teeth than it is moved outwardly in the spaces between the die teeth. This creates two peaks of work material at the roots of each pair of die teeth (i.e., at the top land of the formed tooth); and in the final stages of most of these roll-forming operations, these peaks fold over to form a seam in the top land of each rolled tooth. Such seams are usually displaced slightly from the center of the top land and are inclined due to the direction of tooth sliding during the roll-forming operation.
The resulting seam often causes the outside cylindrical circumference of the gears to be uneven, and it also creates a potential weak spot that can cause a failure of the formed tooth under certain types of heavy loading. Because of these just-recited problems, roll-formed gears have not been considered appropriate for use in automotive differentials.
In modern automotive differentials, the spur and helical gears are configured to provide maximum tooth strength in minimum space, and the design of the meshing gears utilizes the full addendum of each tooth. That is, the highest point single tooth loading ("HPSTL") on each gear tooth usually occurs at the top of its addendum (i.e., at its intersection with the top land of the tooth). Such HPSTL stress, supported by the seamed top lands of roll-formed teeth, greatly increases the chance-of tooth failure, as is explained in greater detail below.
Further, parallel-axis gear differentials often support the gears in housing pockets. In such arrangements, the top lands of the gears act as journals, and the cylindrical inside diameters of the differential housing pockets act as the bearing surfaces for the cylindrical outside diameter of the gears. Any unevenness (or any departure from desired cylindrical form) in the top lands of the roll-formed gears can result in point contact between the outside diameter of the gears and the cylindrical bearing surface of the differential pocket, causing undesirable wear and noise.
Therefore, it has been long assumed that roll-formed gears could not be safely incorporated in differentials. In other words, roll-formed gears have been heretofore considered inappropriate for use in differentials.
Our invention is directed to the solution of these problems and to facilitating the use of an economical roll-forming process in the manufacture of spur and helical gears for automotive differentials, particularly for parallel-axis differentials in which the gears are supported in cylindrical pockets.